Core D: Control Identification Core. Appropriate control selection is an important determinant of the quality of an epidemiologic study. For our case-control studies of HT and cancer in Los Angeles County we have chosen to use neighborhood controls selected by following a predetermined algorithm which proceeds through an obligatory sequence of adjacent houses and residential units, beginning at a specific residence which has a specific geographic relationship to the residence where an interviewed case lived at diagnosis. We have developed a detailed manual of procedures for this activity. The timeliness and accuracy of the Field Representatives who conduct this neighborhood canvassing, are monitored and evaluated through a series of quality control procedures, as well as regular reports of activities to the Core Director, and to individual Project Pis. Using this strategy we routinely identify and interview controls for over 90% of interviewed cases. This Core will be used by Projects A and B in this competing application.